It is often desirable to detect magnetic substances which are present in small quantities such as magnetic ink or a paramagnetic gas such as oxygen. For instance, an automatic banknote recognition device may read and or detect an image written or printed with magnetic ink on banknotes of countries including the United States, Japan or Germany. Alternatively, a gas analyzer may detect the presence of a paramagnetic gas. Additionally, it is often desirable to measure the rotational speed of a toothed wheel made of a magnetic material or at least the speed of the teeth and teeth intervals which are coated with a magnetic material.
U.S. Pat. No. 4,864,238 discloses a device for measuring low-flux magnetic fields of the type produced by the magnetized magnetic ink found on banknotes. This patent discloses an extremely sensitive sensor, functioning on the core saturation principle, for measuring the extremely weak magnetic fields produced by the magnetized ink of the banknote. Such magnetic fields typically have a value in the order of 10.sup.-3 Gauss. In comparison, the Earth's magnetic field intensity is 0.5 Gauss or approximately three orders of magnitude greater than the fields of magnetized banknote ink. As such, the above-mentioned sensor requires extensive and costly magnetic shielding to accomplish its task. Furthermore, the banknote must be magnetized by a strong permanent magnet before being read.
It is the object of the present invention to provide a magnetic sensor for which no permanent magnet and no costly and extensive magnetic shielding against foreign magnetic fields is required. It is a further object to provide a sensor capable of static measurement, at least in detecting the presence of the magnetic substance, where the static measurement covers most of the overall image of the magnetic substance (e.g., most of a printed image on a banknote without relative movement between the magnetic substance and sensor). Such a feature would afford greater security against misuse.